Variable frequency switching inverters are often desirable for controlling the speed of alternating current (AC) motors of the induction type as well as brushless direct current (DC) motors. Such switching inverters generally provide pulse width modulation (PWM) power to the motor armature.
The simplest way to generate appropriate PWM power is by using the so-called intersection method or intersection technique, wherein PWM comparators in the switching inverter compare a sine wave control signal to a triangle wave carrier signal. When the value of the control signal is greater than the value of the carrier signal, the PWM comparators turn on, creating a PWM signal high state. When the value of the control signal is less than the value of the carrier signal the PWM comparators turn off, creating a PWM signal low state.
Generation of such analogue signals is possible with digital circuitry using appropriate digital-to-analogue converters, but the use of digital-to-analogue converters is not feasible in some applications. Alternatively, it is possible to generate values of the needed analogue control and carrier signals as a function of time using purely digital circuitry, such as by a microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC) or field programmable gate array (FPGA).
Typical PWM waveform generation implementation requires a carrier signal frequency of 25 kHz or more and the synthesis of such waveforms involves complex trigonometric and floating point arithmetic operations. A high processor clock rate is required to meet computational and interrupt latency times, which increases current demand for the processor in addition to making the processor susceptible for EMI interference. If application cost constraints require a fixed point processor for the PWM waveform implementation, emulation of the required floating point calculation requires more instruction and processing time. This increases application code size and execution times.